But Polish authorities say they have plenty of evidence against Karkoc, including Nazi pay stub records, testimony from other unit members and Karkoc’s own Ukrainian language memoir in which he places himself at the site of at least one mass murder.

Karkoc did not reveal his Nazi past when he applied for citizenship in the United States in 1949, or that he served with the Ukrainian Self Defense Legion. Affiliation with either group would have barred his entry.

“You would think that we know everything we need to know about that period but the reality is that we’re still learning new things almost every day,” said Mark Weitzman of the Simon Wiesenthal Center, a Jewish human rights organization.

Germany investigated Karkoc’s case four years ago, but declined to prosecute after finding him mentally incompetent to stand trial.

So far, there has been no public response from the the Justice Department about whether the U.S. will extradite Karkoc to face trial in Poland.